A Year in the Bones: Investigating Bone Resorption Across a Reproductive Year in Wild Female Assamese Macaques

Reproduction places considerable metabolic demands on females, particularly during pregnancy and lactation, when calcium homeostasis and bone metabolism are challenged. Urinary C-terminal telopeptide of type I collagen (CTX-I) offers a useful non-invasive marker to assess bone resorption. We measured urinary CTX-I in wild female Assamese macaques (Macaca assamensis) living at Phu Khieo Wildlife Sanctuary, Thailand. A total of 663 urine samples were collected from 19 adult females between June 2017 and January 2019 and analysed using a CTX-I ELISA. Generalised additive modelling revealed a non-linear relationship between days relative to conception and urinary CTX-I levels, with higher values before conception and after parturition, and lower levels during pregnancy. However, this pattern remained when lactating females from the previous year (6 out of 19) were excluded, and further analyses showed that the seasonal, rather than the reproductive term, best-explained variation in urinary CTX-I levels. CTX-I levels showed two annual peaks, with the main one occurring in the late rainy season (August-September). Seasonal changes in urine concentration did not account for this pattern. These findings suggest that, in the study population of Assamese macaques, bone resorption varies with season, possibly in response to environmental or endocrine cycles, rather than being primarily driven by the metabolic demands of reproduction.